rustdoc: resolve intra-doc links to builtin macros

[QuietMisdreavus]

Fixes #61804

When rustdoc encounters an intra-doc link that resolves to a built-in macro, it attempts to load up the crate's name as it saves the information about the link target. However, this causes an ICE since the built-in macros don't come from a regular crate. This PR attempts to work around this by handling built-in macros separately, similarly to how it handles primitives - by linking to the standard library's documentation, where there are documentation stubs available to link against.

[Centril]

cc #61877 cc @petrochenkov

[QuietMisdreavus]

Oh yikes, that PR is looking to remove CrateNum::BuiltinMacro entirely. That means this PR would need to find another way to detect that a resolution led to a builtin macro. 😓

[Centril]

@QuietMisdreavus Can you not use is_builtin_macro?

[QuietMisdreavus]

I'd still have to rework the PR - one of the checks in this PR happens after the resolver is gone. And since rustdoc only saves DefIds, the Res::BuiltinMacro alternative discussed in #61877 (comment) wouldn't work as easily, either.

[QuietMisdreavus]

It looks like #61877 was closed, and was replaced by #62086. Looking at the discussion of what it's intended to do, it's possible that that may obviate this PR and fix the linked issue with no work in rustdoc itself.

[petrochenkov]

Closing as superseded by #62086.